1. Field of the Invention
The present invention relates to an electro-optical device and a display device. In particular, the present invention relates to an electro-optical device and a display device such as a liquid crystal display device having high-speed responsibility with respect to an applied voltage and high light transmittance, for controlling light transmission and interception.
2. Description of the Related Art
A liquid crystal display device is widely used for a display unit of various electronic devices such as a liquid crystal display television, a monitor for a personal computer, a mobile phone, and a digital camera, and has a feature of being thin and light, and of low power consumption.
There are many display types in the liquid crystal display device, and there is known an in-plane-switching (IPS) type (JP 3504159 B) as a representative liquid crystal display type of a liquid crystal display device, which is viewable at wide angles. In the IPS type liquid crystal display device, liquid crystals in a nematic phase are generally used. Those liquid crystal molecules are held in a space sandwiched between two substrates. An electric field is applied to the liquid crystal molecules, their orientation directions are rotated, and their effective light axes are rotated in a plane parallel to the substrates, to thereby control light transmittance of the liquid crystal. Accordingly, display is performed.
In the IPS type liquid crystal display device, various types have been proposed as types in which an electric field is applied to liquid crystal molecules. The most general one is to form a source electrode and a common electrode having an interdigit shape on a single substrate. This type includes a type of forming both the source electrode and the common electrode into an interdigit shape on one substrate and a type of forming one of the source electrode and the common electrode into an interdigit shape and forming the other thereof into a plate shape to arrange them on one substrate via an insulating layer.
On the other hand, with respect to the IPS type liquid crystal display device, there have been proposed recently liquid crystal display devices in which a material changing from optical-isotropy phase to optical-anisotropy phase in response to an applied voltage is used (JP 3504159 B). In such liquid crystal display devices described above, a liquid crystal material having a blue phase, a liquid crystal material having cubic symmetry, and the like are used (Yoshiaki Hisakado etc., “Large Electro-optic Kerr Effect in Polymer-Stabilized Liquid-Crystalline Blue Phases,” Advanced Materials, Vol. 17, No. 1, 96 (2005)), and an electric field parallel to the substrates is applied by the interdigit electrodes as in the IPS type. In those liquid crystal display devices, much higher response characteristics of several microseconds to several ten microseconds are obtained compared with the IPS type in which a conventional nematic liquid crystal is used, and thus a display quality for movie display is expected to be greatly improved. However, in an electrode structure in which an electrode interval between the interdigit electrodes is made smaller than an interval between upper and lower substrates (JP 2007-86205 A), and an electric field above the electrode has a small component parallel to a substrate surface. Therefore, if an optically anisotropic material is applied for a medium layer with the structure, the medium layer above the electrode cannot exhibit the optical-anisotropic property.
However, for example, in a liquid crystal display device which is driven by the interdigit electrodes using a blue phase liquid crystal as a liquid crystal material, even when a transparent electrode is used for the electrodes, light does not pass through a region of the electrode portions. This is partially because a normal electric field intensity to be applied to the liquid crystal molecules which are positioned above the electrodes is weak, and therefore optical-anisotropy is not imparted to the liquid crystal molecules. Further, this is also partially because the electric field above the electrode a small component parallel to a substrate surface and a large component vertical to the substrate surface.
In the liquid crystal display device using the blue phase liquid crystal, polarizing plates are attached to outer sides of two substrates so that polarization axes of the respective polarizing plates are at an angle of about 45 degrees with respect to a longitudinal direction of the interdigit electrodes which are aligned parallel to each other. Optical-anisotropy of the liquid crystal molecules is imparted in a direction of an electric field. In a region between the electrodes, the electric field is applied substantially parallel to the substrate surface. When the liquid crystal molecules positioned in the region are applied in the electric field, optical-anisotropy is imparted in the direction of the electric field, which is a direction of 45 degrees with respect to the respective polarizing plates. Hence, light which has entered from the outside is transmitted through the region. However, the electric field applied to a region immediately above the electrode has a weak electric field intensity, and has a larger component vertical to the substrate than one parallel to the substrate. Therefore, the optical-anisotropy imparted to the liquid crystal molecules positioned in the region is small and the optical-anisotropy is in the direction vertical to the substrate, and hence an optical axis of the light which has entered from the outside cannot be rotated, whereby light cannot be transmitted.
FIG. 18 is a vertical cross-sectional view of an electro-optical device according to a related art. FIG. 19 is a plan view of the electro-optical device according to the related art when viewed from an arrow A of FIG. 18. A medium layer 203 is sealed between a substrate 101 and a substrate 103, and an electrode 105 and an electrode 106 are formed on the substrate 101. Polarizing plates 104 and 207 are respectively attached to outer sides of the substrate 101 and the substrate 103. Polarization axes of the respective polarizing plates are at an angle of about 45 degrees with respect to a longitudinal direction of the electrodes. A relative relationship between an applied voltage and transmittance in a case where an alternating voltage is applied from an external power source (not shown) to the electrodes 105 and 106 is illustrated by (b) of FIG. 20. The electro-optical device according to the related art has an extremely low light transmittance, and therefore cannot be adapted to an actual device. As a result of observation with a microscope, in the electro-optical device according to the related art, light is not transmitted at all through a region above the electrode having an interdigit shape.
Unlike the nematic liquid crystal, the blue phase liquid crystal has an extremely short interaction length between liquid crystal molecules. Accordingly, in the liquid crystal display device using the blue phase liquid crystal, even when optical-anisotropy is generated in liquid crystal molecules positioned in a region between the electrodes, the influence of the optical-anisotropy does not extend to the liquid crystal molecules in a region above the electrode.
For those reasons, the optical-anisotropy is not imparted to the liquid crystal molecules positioned in the region above the electrode, and hence light is not transmitted through the region above the electrode, whereby the entire transmittance is reduced. Therefore, it is necessary to increase luminance of a backlight so as to obtain appropriate luminance for a liquid crystal display device, which leads to a problem in that power consumption is increased. When a liquid crystal display device using an optically-anisotropic liquid crystal material which reacts to an applied electric field at high speed, such as the blue phase liquid crystal, is put into practical use, it is indispensable to increase the light transmittance of the liquid crystal so as to obtain a clear image.
Further, in the liquid crystal display device using the blue phase liquid crystal and the interdigit electrodes, there arises a problem that an electric field from a voltage at which the maximum transmittance is attained is several V/μm, which is extremely high, and the voltage is about ten times as high as or higher than a drive voltage for the IPS type liquid crystal display device using the nematic liquid crystal. As a result, a drive switching device which is driven at low voltage, such as a thin film transistor, cannot be used for a drive switching device therefor.
Further, as a result of evaluating characteristics of the display device based on known information, the applied electric field has a small component parallel to the substrate surface in a region above the source electrode or the common electrode, and hence light is not transmitted through the region. Even when the electrode is transparent, the fact scarcely contributes to a screen display.